1. Field of the Invention
The invention relates generally to systems for aiming directional antennas and more particularly to a method and apparatus using a global positioning system (GPS) receiver and a compass for automatically aiming a directional antenna to a distant geographical or spatial location.
2. Description of the Prior Art
Most antennas that are used on boats, road vehicles, airplanes, and other mobile platforms necessarily have a wide radiation pattern in order to be able to receive or transmit signals without regard for the directional orientation and geographical or spatial location of platform. For example, marine and vehicle television antennas typically have radiation patterns of 360.degree. in a horizontal plane for receiving television signals from terrestrial television transmitters. Such television antennas are limited by the wide radiation pattern to having a low gain. When the boat or vehicle is near to the television transmitter, the low gain of the antenna is not noticed because the signal-to-noise ratio for the signal received from the transmitter is high. However, when the boat or vehicle is farther away from the television transmitter where the signal-to-noise ratio is lower, the low gain of the antenna degrades or even prevents television reception. Of course, a high gain directional antenna, such as is commonly used in a residence, could be used to increase signal-to-noise. However, such directional antennas must be aimed toward the television transmitter. Each time the platform rotates or the platform moves so that the direction between the antenna and the transmitter changes, the direction of the antenna must be adjusted correspondingly. For receiving satellite television, the limitations are more severe because satellite television signals generally have low signal-to-noise ratios everywhere on the Earth's surface. Common residential satellite television antennas use parabolic dish reflectors that are highly directional and have very high gains in order to compensate for the low signal level of satellite television signals. However, a non-directional marine or vehicle satellite television antenna requires an upward pointing hemispherical radiation pattern for receiving television signals from a satellite transmitter. The hemispherical pattern for satellite reception is even broader than the 360.degree. horizontal pattern for terrestrial reception, resulting in an even lower antenna gain for the satellite television antenna. It is unlikely that a low gain hemispherical antenna could provide enough signal strength for receiving satellite television. Similar limitations exist for other types of signals and for transmitting from a mobile platform as well as receiving.